The normal techniques of preservation by pasteurization or sterilization aim to make products preservable for a long period of time whilst trying to keep their taste and their appearance.
The main difficulty resides in the fact that the products are normally heated by direct contact, that is to say by conduction or by convection and most often at high temperatures and for long periods of time, which makes the product lose some of its organoleptic and physico-chemical properties.
By way of example, let us consider the particular case of techniques for preserving fruit. For most known techniques relating to the preservation of fruit, the heat treatment of the pieces of fruit produces a puree or stewed fruit which is not suitable for most current industrial or household uses, in particular for cake making.
Various methods for preserving or cooking foodstuffs, which attempt to remedy this problem are known in the state of the art.
In particular, document FR-A-2,635,167 describes an apparatus for steam treatment of products in a pressurized or vacuum chamber. Although the products are apparently treated in bulk, it seems to be necessary to arrange them, for example, on trays or trolleys in order to carry out the processing in the apparatus.
Furthermore, this processing can in no way be carried out continuously or semicontinuously. In fact, the apparatus has to be stopped in order to introduce the foodstuffs into the pressurized or vacuum chamber.
Document EP-A-0,006,369 describes a method for continuous heat treatment of unpackaged or packaged bulk products in a casing in which a vacuum is set up; this vacuum is then broken by an external supply of steam. This step has the consequence of pressurizing the casing.
Document CA-A-975,976 describes a method for cooking foodstuffs according to which water is heated to boiling in an enclosed container, and air and some of the steam created in this container are expelled so as to obtain a partial vacuum therein, the cooking is carried out under partial vacuum and this partial vacuum is terminated before removing the foodstuffs from the container. This method which is described obviously does not allow continuous or semicontinuous treatment. Again, the machine should be completely stopped before extracting the foodstuffs.
Document U.S. Pat. No. 4,543,283 describes an apparatus and a method which are intended for heating and cooling foodstuffs continuously, in which the steam treatment container is under pressure.